Why is this blue color appearing
The problem appears to be due to negative values being used in the Mix factor resulting in artifacts where the 'negative' of the orange colour is becoming evident.
Each channel from the Normal is in the range -1.0 through to +1.0 and so the 'add' can result in negative values. Normally a 'mix' factor will automatically clamp the values at 0.0 and 1.0 but in this case that doesn't seem to be happening. By enabling 'Clamp' you are limiting the output of the Add maths node to strictly the 0.0 to 1.0 range so that the mix is behaving consistently.